The invention is based on an apparatus and a method for calibration of an optoelectronic sensor, the sensor at least intermittently also receiving UV light.
The invention is further based on an apparatus and a method for mensuration of features on a substrate using a UV microscope and a spatially resolving optoelectronic sensor.
Optoelectronic sensors convert light into electrical signals and are therefore used in science and technology to detect and measure light. In cameras, spatially resolving optoelectronic sensors are used for image acquisition.
The sensors on the one hand have a dark current, i.e. even when no light is incident, they release electrons and emit an electrical signal. On the other hand, they exhibit a saturation behavior when they are illuminated with a sufficiently large light quantity. The response characteristic of the sensor extends within these boundaries. The response characteristic is substantially linear within a certain range, so that the electrical signals of the sensor are proportional to the light quantity received by the sensor.
Most of these sensors are sensitive in the visible and infrared light wavelength region. In these wavelength regions, their sensitivity and response characteristics to the acquired light do not change.
It is known, however, that UV light can influence and reduce sensor sensitivity. UV light can generate electron-hole pairs that modify the lattice structure of the sensor. Quantitative information about this is not present in the sensor manufacturers' data sheets, however, neither in general terms nor, especially, for the particular individual sensor. Changes in sensor sensitivity due to UV irradiation obviously are of no consequence for most applications. For consumers using cameras for imaging in natural ambient light, the camera's optical system is also not designed for UV light, so that the sensor also does not receive the UV component in natural ambient light.
Special sensors that can also detect UV light are available commercially. Their spectral specification generally lies in the wavelength region between 200 nm and 800 nm. The response of such a sensor, however, i.e. the electrical output signal of the sensor as compared to the incident light quantity, is much lower in the UV region than in the visible wavelength region, and is equal to only a few percent.
UV-capable sensors of this kind have a number of technical applications for measurements in the UV region. On the one hand, material analyses can be made using UV light. In the case of semiconductors, for example, information is required about material composition and optical properties. The refractive indices, absorption coefficients, and thicknesses of layers applied onto semiconductor wafers need to be determined. In particular, very thin layers can be measured more accurately with UV light than with visible light. It is necessary in this context that the signals be stable and also reproducible at a later point in time. The reproducibility of the measurements improves the material analysis, and the results are more comparable with previous results. Such measurements are accomplished by reflection at the specimen, and are performed over a wavelength range. Spectrophotometers and/or spectroellipsometers are used, in particular, for this purpose.
On the other hand, UV sensors are also used for UV imaging. The resolution of the images is improved by the use of UV light. In addition, specimen features look different under UV light, and yield additional information as compared to visible-light images. As a result, defects and very small particles on specimen surfaces of, for example, semiconductor substrates can be better detected and classified. Feature spacings and feature widths can moreover be ascertained by image processing applied to such UV images. The accuracy of the measured spacings and widths (critical dimension (CD) measurements) can be improved by means of images acquired in the UV region.
It has been discovered that especially in the context of stringent CD measurement accuracy requirements, the stability of the measurements is not sufficient. A drift of measured results in one direction is observed when specific features on a substrate are measured repeatedly over a period of time. That period can often be only a few hours. It has been found that the cause of this drift in the measured results lies in the exposure of the sensor to UV light. The exposure to UV light apparently causes permanent changes in the sensor in terms of its optoelectronic properties.